The present invention relates to a process for reacting an organic compound with a hydroperoxide using a heterogeneous catalyst wherein both the pH and the temperature of the reaction medium are changed during the reaction. In a further embodiment, the present invention relates to a process in which the pressure under which the reaction occurs is changed in addition to the pH and the temperature of the reaction medium.
In reactions of organic compounds with a hydroperoxide in the presence of a heterogeneous catalyst, the activity of the heterogeneous catalyst generally decreases on prolonged use.
To be able to reuse the catalyst, it is in these cases necessary to remove the catalyst from the reactors in which the reactions are carried out and regenerate it outside the reactors. In the case of heterogeneous titanium silicalite catalysts which are used, for example, for the reaction of olefins with hydroperoxide solutions, such regeneration processes are carried out, for example, by calcination of the catalyst at elevated temperature, as described, for example, in J. Catal. 129 (1991) 159-166. Another possibility, which is likewise described in this publication, is to wash the catalyst with suitable solvents. Since regeneration by these methods can only be carried out after the catalyst has been removed from the reactor, these processes are complicated and therefore undesirable from the point of view of process economics. Further regeneration methods are described, for example, in WO 98/55228 and the prior art cited therein.
To counter the gradual deactivation of heterogeneous catalysts which are used in the reaction of organic compounds with a hydroperoxide, it is also possible to regulate the temperature during the reaction so as to compensate for the deactivation. It is likewise conceivable for temperature and pressure to be changed during the reaction in order to compensate for the deactivation of the catalysts. This method is described, for example, in WO 99/01445, where temperature and pressure are simultaneously increased during the reaction of propene with an active oxygen species in the liquid phase over a heterogeneous catalyst.
A further possible way of influencing the selectivity of, for example, titanium silicalites is described in J. Catal. 140 (1993) 71-83. It is found there that, without quantitative results being reported, the addition of alkali metal hydroxides in the epoxidation of lower olefins in low concentrations increases the yield, but does not influence the selectivity. However, at relatively high base concentrations, the activity of the titanium silicalite may be completely suppressed. The epoxidation of allyl chloride, which is described in the same document, is significantly impaired by the addition of neutral salts such as LiCl when using a titanium silicalite, while the addition of HCl improved the activity of the catalyst. In contrast, the addition of the neutral salt LiCl in the epoxidation of 1-butene does not adversely affect the activity of the catalyst.
EP-A 0 712 852 discloses the use of a nonbasic salt to improve the selectivity of a titanium silicalite catalyst which is used for the epoxidation of olefinic compounds by means of hydrogen peroxide. The experiments were carried out in the batch mode at constant temperatures.
EP-B 0 230 949 discloses a process for the epoxidation of olefinic compounds by means of hydrogen peroxide, in which the selectivity of the catalysts used, viz. synthetic zeolites, can be significantly improved by adding, either before or during the reaction, compounds which neutralize the acid groups on the catalyst surface. The process described was carried out under isothermal conditions.
P-A 0 757 043 describes a process for preparing epoxides from olefins and hydrogen peroxide in the presence of a titanium-containing zeolite as catalyst, in which neutral or acidic salts are added to the catalyst before or during the reaction. The temperatures in the reaction of the olefins with hydrogen peroxide were kept constant.
It is found in practice that the deactivation of the catalyst often does not occur uniformly. Rather, the initial activity, which is generally very high, decreases very quickly. This is followed by a relatively slow deactivation which can extend over some hundreds of hours.
It is an object of the present invention to provide a process which allows a flexible response to these different deactivation rates of heterogeneous catalysts which occur in reactions of organic compounds with hydroperoxides.